Oracle 行转列总结 Case When,Decode,PIVOT 三种方式
最近又碰到行专列问题了,当时不假思索用的是子查询,做完后我询问面试管行专列标正的写法应该如何写,他告诉我说应该用"Decode",索性我就总结一下,一共三种方式

--=========建表语句

CREATE TABLE populations(
country VARCHAR2(20),
sex INT,
populcation INT
);

--=========插入记录

insert into populations (COUNTRY, SEX, POPULCATION)
values ('中国  ', 1, 340);

insert into populations (COUNTRY, SEX, POPULCATION)
values ('中国  ', 2, 260);

insert into populations (COUNTRY, SEX, POPULCATION)
values ('美国  ', 1, 45);

insert into populations (COUNTRY, SEX, POPULCATION)
values ('美国  ', 2, 55);

insert into populations (COUNTRY, SEX, POPULCATION)
values ('加拿大', 1, 51);

insert into populations (COUNTRY, SEX, POPULCATION)
values ('加拿大', 2, 49);

insert into populations (COUNTRY, SEX, POPULCATION)
values ('英国  ', 1, 40);

insert into populations (COUNTRY, SEX, POPULCATION)
values ('英国  ', 2, 60);

commit;

--=========三种方式

--------------------------------------

Decode 方式

SELECT a.COUNTRY AS "国家",
   SUM(DECODE(a.SEX, 1, a.POPULCATION)) AS "男",
   SUM(DECODE(a.SEX, 2, a.POPULCATION)) AS "女"
  FROM populations a
GROUP BY a.COUNTRY

--------------------------------------

Case When 方式

SELECT a.COUNTRY AS "国家",
   SUM(CASE
WHEN a.SEX = 1 THEN
  a.POPULCATION
   END) AS "男",
   SUM(CASE
WHEN a.SEX = 2 THEN
  a.POPULCATION
   END) AS "女"
  FROM populations a
GROUP BY a.COUNTRY

--------------------------------------

PIVOT 方式 11G特有的方式

SELECT *
  FROM (SELECT a.COUNTRY AS "国家", a.SEX, a.POPULCATION FROM populations a)
  PIVOT(
SUM(POPULCATION)
FOR SEX IN(1 AS "男",2 AS "女"))